This invention relates to signal transmission in communication systems in which a transmitter and a receiver communicate over a communication channel using modulated carrier signals. More particularly, this invention relates to control of transmitted power at frequencies outside the channel during such communications. The present invention may be used, for example, in Orthogonal Frequency Division Multiplexing (OFDM)-based wireless local area network (LAN) communication systems.
In a radio communication system, it is often desirable to control the transmitted power, both at frequencies within the channel in which signals are to be transmitted and at frequencies outside the channel. For instance, standards organizations may propose or promulgate communication standards that define channels and set limits on in-channel and out-of-channel transmitted power. An example is the Institute of Electrical and Electronics Engineers (IEEE), which in IEEE 802.11a/b/g/n specifies the physical layer and medium access control for OFDM systems that provide wireless LANs in the Industrial, Scientific, and Medical (ISM) and the Unlicensed National Information Infrastructure (U-NII) frequency bands. As used herein, a “channel” is a predetermined electromagnetic frequency interval available for communication between a transmitter and a receiver, and a “band” is a predetermined electromagnetic frequency interval that contains a plurality of channels having common purposes and common standards and/or regulatory treatment.
Governmental regulatory bodies may also set limits on transmitted power at various frequencies. Such regulatory bodies include the United States Federal Communications Commission (FCC), the European Telecommunications Standards Institute (ETSI), and the Japan Ministry of Public Management, Home Affairs, Posts and Telecommunications (MPHPT).
Various transmitter architectures can be used in systems such as wireless LANs. A transmitter having a superheterodyne architecture can provide high performance, but is generally larger and costlier than one using an alternative architecture. A transmitter having a zero-intermediate-frequency (ZIF) architecture is generally smaller and cheaper than one having a superheterodyne architecture, but generally has worse performance. A transmitter having a low-intermediate-frequency (LIF) architecture, in which the intermediate frequency is less than the modulating frequency or the channel bandwidth, can provide better error vector magnitude and therefore better system performance (lower error rate and/or larger range) than one having a ZIF architecture. However, the finite in-phase/quadrature (I/Q) image rejection in the analog circuit blocks of an LIEF transmitter causes an image in its power spectrum, which can be undesirable.